Isotopic Diversity in Mangaia Melt Inclusions: Mantle Source or Crustal Assimilation?

锰熔体包裹体的同位素多样性：地幔源还是地壳同化？智力优势：含橄榄石的熔体包裹体（MIs）被深部岩浆通道中生长的斑晶所捕获，并提供快照。在熔体完全聚集之前，熔体成分不同。MIs揭示了主要元素和微量元素的多样性，仅从整个岩石熔岩的分析中是不清楚的，这种多样性的起源是一个激烈争论的来源。一端元假说认为，地幔源的多样性反映了地幔源成分的非均质性。另一种可能是岩浆作用的结果，包括地壳的同化作用。在一个案例中，来自Mangaia岛（Cook-Austral群岛）的单一熔岩中含有橄榄石的MIs表现出极端的pb同位素变异，其范围跨越了所有海洋岛屿和MORB熔岩所观察到的一半范围。令人惊讶的是，其中一些MIs中含有的铅比在任何锰熔岩中观察到的放射性低得多。这些数据引起了许多基本问题。为什么包裹体在同位素上不能代表散装岩浆？这是因为单个熔岩样本中的MIs是来自同位素不同的地幔源的一系列熔体吗？或者，同位素多样性反映了岩浆上升过程中岩石圈的同化作用？本文建议对一组新样品进行微量元素和铅同位素组成的测量，以便利用这些综合数据来评价地壳同化和其他导致铅同位素多样性的过程。对Mangaia的14个MIs的初步研究暗示了地壳同化的可能影响，但数据有限，需要额外的数据来最终描述和评估相互竞争的假设。特别是，如果锰海的铅同位素多样性是由地壳同化过程引起的，那么预计铅同位素组成将与同化示踪剂（如高Cl和B、Sr同位素）相关，而如果同位素多样性反映了锰海下不同的地幔来源，则不太可能出现这种情况。为了能够对这些备选方案进行综合评价，要求基金收集新的高精度Pb和sr同位素数据以及微量元素和挥发性数据，以扩大原始Mangaia熔岩的MIs收集。本研究旨在约束岩浆过程和地幔源在非均质包裹体形成过程中的相对作用，并为岩石圈岩浆运移和侵位过程提供新的认识。更广泛的影响：拟议的研究将支持一名早期职业非终身教职员工和一名博士生。该项目还将涉及几名本科生顾问。2010年，在库克群岛进行实地考察时，PI在Mangaia高中向学生讲授当地火山活动，并接受了当地报纸关于拉罗汤加岛的采访，结果发表了一篇文章，描述了他在那里的研究。PI还会见了库克群岛总理，讨论拟议工作的相关性。

Isotopic diversity in Mangaia melt inclusions: Mantle source or crustal assimilation?Intellectual Merit: Olivine-hosted melt inclusions (MIs) are trapped by growing phenocrysts in magma conduits at depth and provide ?snapshots? of diverse melt compositions before complete melt aggregation. MIs reveal major and trace element diversity that is not clear from analyses of whole rock lavas alone and the origin of this diversity is a source of vigorous debate. One end member hypothesis is that the MI diversity reflects heterogeneity in mantle source compositions. Alternatively, MI diversity may result from magmatic processes, including crustal assimilation. In one case, olivine-hosted MIs from a single lava from the island of Mangaia (Cook-Austral Islands) exhibit extreme Pb-isotopic variability spanning half the range observed in all ocean island and MORB lavas. Surprisingly, some of these MIs contain Pb that is significantly less radiogenic than observed in any Mangaian lavas. These data give rise to many fundamental questions. Why is it that the inclusions are not isotopically representative of the bulk magma? Is this because MIs from a single lava sample an array of melts derived from isotopically-diverse mantle sources? Or, does the isotopic diversity reflect assimilation of the lithosphere during magma ascent? It is proposed to measure trace element and isotopic compositions of MIs in a new set of samples from Mangaia so that these comprehensive data can be used to evaluate crustal assimilation and other processes responsible for the Pb isotopic diversity. Preliminary study of 14 MIs from Mangaia hint at a possible influence of crustal assimilation, but the data are limited additional data are required to conclusively describe and assess the competing hypotheses. In particular, if the Pb isotopic diversity in Mangaia MIs is caused by crustal assimilation processes, it is expected that Pb-isotopic compositions will correlate with tracers of assimilation (e.g., high Cl and B, Sr isotopes), whereas this is unlikely to be the case if the isotopic diversity reflects diverse mantle sources beneath Mangaia. To enable comprehensive evaluation of these alternatives, funds are requested to collect new high-precision Pb- and Sr-isotopic data as well as trace element and volatile data for an expanded collection of MIs from primitive Mangaia lavas. The proposed research aims to constrain the relative roles of magmatic processes and mantle sources in generating heterogeneous inclusions, and will provide insights into magma transport and emplacement processes in the lithosphere.Broader Impacts: The proposed study will support an early-career non-tenured faculty and one PhD student. The project will also involve several undergraduate advisees. While carrying out fieldwork in the Cook Islands in 2010, the PI taught students about local volcanism at the Mangaia high school and interviewed with the local newspaper on Rarotonga, resulting in an article that describes his research there. The PI also met with the Prime Minister of the Cook Islands to discuss the relevance of the proposed work.